Set

Definition

A set is a collection of distinct objects, considered as a single mathematical entity. The objects in a set are called elements or members.

Elements of a set are

• Unordered ($\{1,2,3\}$ is the same as $\{3,1,2\}$)
• Distinct elements ($\{1,2,2,3\}$ is the same as $\{1,2,3\}$ )
• Well-defined: There must be a clear criterion for whether an object belongs to the set or not.